Most smokers want to quit and nearly half attempt to quit each year. However, few actually succeed. The problem is particularly acute among young adult smokers who tend to under-utilize existing smoking cessation services, and have limited access to health insurance and healthcare. New, innovative approaches are needed that can reach out to younger adult smokers and help them quit. Intervention delivery modalities that can be inexpensively delivered in an appealing format with wide reach are particularly compelling for treating younger smokers. For this project we will develop and test a theoretically driven, evidence-based smoking cessation counseling intervention that can be delivered through SMS text messages. Text messaging is popular with younger adults (<35 years), over half of whom use text messaging, often sending 50 or more messages per week. Text messaging can be used to provide advice and interactive support adapted from evidence-based interventions for smoking cessation. However, thus far text messaging is an untapped medium and has been only rarely studied as an intervention delivery tool. Existing studies have significant limitations which the proposed study will attempt to redress. This project consists of three sub-studies: Initial Acceptance Testing, Prototype Evaluation, and initial Efficacy Testing. We will first develop a detailed presentation of the proposed intervention system and conduct an initial test of the program's potential feasibility and acceptability using a series of 3-4 focus groups with 24 smokers from our targeted demographic and conduct semi-structured interviews with key stakeholders. Information obtained from these interviews and focus groups will be used to inform prototype development. Intervention programming and the content of intervention messages will be completed in month 9 of the project period. We will then recruit a second sample of 24 adult smokers to use the program as a smoking cessation intervention for 7 weeks, and then participate in focus groups to evaluate their experience with the system. We will also demonstrate the working prototype to key stakeholders and conduct interviews to obtain their feedback. Based on these evaluations, we will refine the intervention. In year 2 we will recruit a sample of 40 adult smokers from the target group and randomly assign them to receive either the text-message intervention (TXT) or a self- help intervention with contact control (TXC). Outcomes at end-of-treatment and six-month follow up will be used to obtain effect size estimates needed for a larger trial. PUBLIC HEALTH RELEVANCE: Younger adult smokers typically have limited access to healthcare and under-utilize smoking cessation services. This project will develop an evidence-based, theoretically driven smoking cessation program delivered through text messaging, which is a popular form of communication used by the majority of younger adults. This project has substantial public health significance in that successful dissemination of the intervention could reach as many as 6.8 million smokers annually.